The present invention relates to a multi-pulse laser beam generation method and a device therefor and a laser beam machining method and an apparatus using a multi-pulse laser beam, and more particularly to a method and a device for converting one laser-pulse beam into two-wavelength multi-pulse laser beams and a laser beam machining method and an apparatus using a two-wavelength multi-pulse laser beam.
As a conventional multi-pulse laser beam generator, a delayed pulse laser beam generator has been disclosed in JP-A-62-123788 (NEC) laid open on Jun. 16, 1987. The delayed laser beam pulse generator includes first and second polarizers of similar characteristics for transmitting or reflecting an input laser beam of circular polarization to split the input laser beam into P-polarized and S-polarized beams, a polarization rotation device disposed between these polarizers for rotating by 90 degrees the polarized laser beam of the P-polarization or S-polarization reflected by the first polarizer, and a total reflection mirror for forming an optical path for a laser beam, and converts a laser beam of circular polarization into a laser beam of linear polarization having two mutually orthogonal axes with a delay time, to thereby provide a delayed pulse laser beam. In such a case, pulse beams to be outputted to the outside of the device are limited to two kinds of beam. The prior art device does not use a half-mirror to prevent loss of laser beam energy.
A laser beam machining is suitable for application to a multi-layered board including metal thin layers like conductor wiring layers. It is general that drilling is applied to provide holes on a printed circuit board laminated with metal thin layers and resin layers.
A drilling method to provide holes has been widely distributed because the method has an advantage of batch processing a laminated board. However, as the circuit structure on the printed circuit board has become highly packed a drilling diameter has also become smaller, raising a problem of poor durability of holed printed circuit boards. Particularly, in the case of a large circuit board used for a large computer, a lot of drilling work is required and it becomes essential to carry out periodical changing of drills for avoiding occurrence of faults on a single circuit board due to damage of the circuit board by a poor drilling work.
A laser beam machining method has recently been studied to replace the above-mentioned drilling method. The laser beam machining method not only eliminates poor circuit boards, but also can cope with providing holes of small diameters such as about 0.3 mm which is difficult to achieve by the mechanical drilling work. Further, the laser beam machining method has an advantage of not applying physical force to a printed circuit board because of its non-contact processing. However, it is difficult to carry out an efficient processing work of a laminated board by using only one laser beam of one wavelength obtained by one laser unit, because a laminated board including laminated layers of different materials such as metal thin films and resin layers has different absorption coefficients of laser beams. Accordingly, two laser beams of two different wavelengths obtained from two different laser units must be used to solve this problem. Examples of this type of device are disclosed in JP-A-01-266983 laid open on Oct. 24, 1989, JP-A-63-273587 laid open on Nov. 10, 1988 and JP-A-01-273684 laid open on Nov. 1, 1989. Techniques relating to laser beam machining are laid open by the JP-A-54-120498, JP-A-63-136546 and JP-A-63-154280.
While the above-described prior art techniques are processing methods taking advantage of characteristics of different laser wavelengths, they have a problem that an overall structure of a device becomes complex and is large-scaled because two completely different types of laser units are used in the system.